With an increase in the population of aged people, the number of aged patients having senile cataract has noticeably increased. For treating a patient with cataract, the opaque lens and cortex are removed and the vision is corrected with an ophthalmic lens or a contact lens, or an intraocular lens is inserted. It is a generally practiced method at present to remove the entire lens and then fix an intraocular lens in the capsule.
In recent years, as ultrasonic emulsification suction has come to be widely used, an intraocular lens that is insertable through a small incision has been developed and widely clinically used for reducing post-surgery astigmatism and surgery invasion. This lens is a soft intraocular lens that can be inserted through a small incision since its optic portion is formed of a soft material and is hence foldable.
Various inventions have been so far made for materials for intraocular lenses excellent in transparency and flexibility. For example, there have been proposed an intraocular lens obtained by copolymerization of a mixture containing at least two (meth)acrylate monomers having aromatic rings and a crosslinking monomer (see JP-A-4-292609), an intraocular lens obtained by copolymerization of a mixture containing a perfluorooctylethyloxypropylene (meth)acrylate monomer, a 2-phenylethyl(meth)acrylate monomer, an alkyl(meth)acrylate monomer and a crosslinking monomer (see JP-A-8-224295), an intraocular lens formed of a copolymer from a monomer of which a homopolymer has a refractive index of 1.5 or more, a monomer of which a homopolymer has a glass transition temperature of less than 30° C. and a crosslinking monomer, and the like (see Japanese Translation Version No. 8-503506 of PCT Application).
All of these intraocular lenses have transparency and flexibility and are deformable, so that they can be inserted through a relatively small incision. After such an intraocular lens is implanted in the eye, however, a so-called glistening phenomenon takes place, in which the transparency of the intraocular lens is greatly decreased or removed due to numerous small and large water bubbles that are thought to be caused by a phase separation of water that has infiltrated into a lens material, from the lens material.
For overcoming the above glistening problem, there have been proposed a copolymer formed from only one main aryl acrylic hydrophobic monomer and one main hydrophilic monomer present in an amount that is not larger than the amount of the former (see JP-A-2001-316426) and a soft intraocular lens that is formed of a polymer obtained by polymerization of a monomer component containing a hydrophilic monomer and that has a water absorptivity of 1.5 to 4.5% by weight (see JP-A-11-56998).
It can be thought that these materials can inhibit the phenomenon of glistening, etc., which occurs after the implantation of an intraocular lens in the eye. Since, however, a hydrophilic monomer is contained, the refractive index is decreased. Further, when a hydrophilic monomer such as 2-hydroxyethyl methacrylate (“HEMA” hereinafter) is used, the phenomenon of calcium deposition takes place, which may possibly cause the opacification of a lens. Further, when a hydrophilic monomer such as N-vinylpyrrolidone is used, the polymerization reactivity may be decreased and an unreacted monomer may be eluted.
Further, there is proposed an intraocular lens material that is a copolymer obtained by polymerization of a monomer mixture containing 2-hydroxy-3-phenoxypropyl acrylate as a monomer component and a crosslinking agent (JP-A-8-173522).
The copolymer described in JP-A-8-173522 exhibits low tackiness (low adhesion), a low water content and high refractivity. However, the present inventors have ascertained that this copolymer is insufficient for inhibiting the glistening phenomenon although it contains, as a structural unit of the copolymer, a unit derived from a hydroxyl-group-containing monomer such as 2-hydroxy-3-phenoxypropyl acrylate (see Comparative Example to be described later).